These instructions are for Debian-based distros and were tested with Debian 10 and Ubuntu 18.04. For others it should be similar.
- Install Mono. The `mono-complete` package from the Debian repo doesn't include `msbuild`, so you have to install `mono-complete` by following the instructions on the Mono project's website: https://www.mono-project.com/download/stable/#download-lin
- Go to a directory of your choice and clone the repository (it'll automatically create an `UltimateDoomBuilder` directory in the current directory): `git clone https://github.com/jewalky/UltimateDoomBuilder.git`